Method and device for reducing beam current modulation caused by mechanical vibrations in a TWT

ABSTRACT

A method and a device for reducing the modulation of the beam current in a travelling wave tube caused by mechanical vibrations, in which the modulation of the beam current is measured and compared with a reference value. The difference signal generated by the comparison affects the grid bias of the travelling wave tube in such a way that the modulation of the beam current is opposed.

BACKGROUND

The present invention relates to a method and a device that reduces theinfluence of the mechanical environment on the electrical properties oftravelling wave tubes.

The Travelling Wave Tube--TWT--is a wideband amplification tube forsignals within the microwave range. It is used, for example, in certaintypes of radio links and in radar stations. In transmission-coherentdoppler radar systems it is usual to utilize grid pulsed TWTs astransmission pulse amplifiers. In these systems the transmitted spectralpurity of the signal is of great importance for, among other things, theability of the system to suppress clutter signals. A TWT is howeverinfluenced by the mechanical vibration environment and in particularlyexposed environments, as in aeroplanes or target tracking missiles,influence of vibrations can seriously impair the performancecharacteristics of the system.

The source of the problems is when the TWT is subjected to mechanicalvibrations, movements between the electrodes of the electron gun willoccur. These movements result in the current beam through the tubereceiving a modulation at the vibration frequency. For frequencies wherethere exist mechanical oscillatory resonances in the electron gun, thecurrent modulation becomes considerable.

The current modulation caused by the vibration leads to the microwavesignal amplified in the TWT obtaining an amplitude and phase modulationthat is proportional to the current modulation. The microwave signalwill therefore obtain side bands located at a distance equal to thevibratory frequency on both sides of the carrier frequency. In radarsystems this gives rise to deteriorated clutter suppresion for targetspeeds that, from a doppler point of view, correspond to the vibrationfrequencies. The usual way to get around the problems of currentmodulation is to isolate the tube, or the complete transmitter, or eventhe complete radar station, from vibrations. Such measures are howevercomplicated and voluminous and do not always give a satisfactory result.In certain applications, such as target tracking missiles, it is, from aspace point of view, hardly possible with such measures.

In certain applications it can be possible to avoid these difficultiesby replacing the TWTs with other components that are less sensitive tovibrations with regard to spectral purity. Injection locked magnetronsor cross field amplifiers can, by way of example, be used. Thesecomponents, on the other hand, have inferior high frequencycharacteristics with respect to bandwidth, pulse formation and noise anddue to this, other system performance characteristics deteriorate aswell.

SUMMARY

An object of the present invention is to reduce the negative influenceon the electrical performance characteristics of a TWT, that occur dueto the modulation of the current beam by the mechanical vibrations thatthe tube is subjected to, but without using space-demanding mechanicaldevices for vibratory isolation.

Said object is achieved by means of a method and a device, by means ofwhich the modulation of the current beam is measured and compared with areference value. The difference signal that is formed during thecomparison, influences the grid bias of the TWT and accordingly the beamcurrent, so that its modulation is counteracted.

The modulation of the beam current is measured by measuring the cathodecurrent or collector current of the tube, measuring the modulation ofthe high frequency signal that has been amplified in the tube etc.

By making use of electrical signals for measurement of modulation andfor control of the TWT the invention eliminates the need forspace-demanding mechanical devices for vibratory isolation and achievesa solution that can also be applied in devices with limited availablespace.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a device according to the invention.

FIG. 2 shows a block diagram of another device according to theinvention.

DETAILED DESCRIPTION

With reference to FIG. 1 the invention will now be described in the formof an exemplary embodiment.

Reference numeral 1 in the drawing denotes a Travelling Wave Tube (TWT).The tube is provided with a collector connection 2, a cathode connection3, an anode or body connection 4 and a grid connection 5. The helixstructure (or other equivalent structure) 6 of the tube is also shownsymbolically. The TWT can also be provided with more collectors, but asthis has no significance for the present invention, only one collectoris depicted in the drawing.

Those voltage sources necessary for the operation of the tube areconstituted by a collector voltage source 7, connected between thecollector connection 2 and, via the block 9, the cathode connection 3.If the TWT has more collectors, then each one of these are connected toa voltage source. A cathode voltage source 8 is connected between thebody connection 4 of the tube and, via the block 9, the cathodeconnection 3. The body connection of the tube is in most cases"grounded". Further there is a grid voltage source 10 which, via block 9and block 11, is connected between the cathode connection 3 and the gridconnection 5 respectively. The block 11 is constituted by a device thatinfluences (modulates) the grid voltage in dependence upon a controlsignal from a control device (not shown), connected to the point 12. Thecontrol signal can, by way of an example, be a pulse train whereby thecurrent through the TWT (and therefore also its output signal) can bepulse modulated. In certain applications even the collector is earthed,whereby the cathode and collector voltage sources can be replaced by acommon voltage source.

The function of the travelling wave tube is well known for a man skilledin the art and will therefore only be briefly touched upon. In the tube,a well focused electron beam that travels through a wire spiral, a socalled helix, is generated by an electrode system (electron gun). Thefundamental property of the travelling wave tube is, by means of aninteraction between the electron beam through the helix and a signal("wave") that propagates alongside it, to transfer the kinetic energy ofthe electrons to the wave that is thereby amplified. It should bepointed out that the helix can be replaced by a series of connectedcavities. This is mostly common in tubes intended for high power.

As was mentioned in the beginning, the electron beam can be modulated bymeans of external influence, for example mechanical vibrations. Thismodulation will influence the wave, due to an interaction between theelectron beam and the wave. The mechanical vibrations will thus cause anundesired modulation of the amplified signal.

Since the current--the beam current--which the electron beam represents,constitutes a part of the current through the tube, the modulation ofthe electron beam can be measured as modulation of the current--cathodecurrent 13--which passes through the tube. The cathode current iscomprised mainly of two components: the body or helix current 15 throughcathode voltage source 8 and the collector current 14 through collectorvoltage source 7.

As is evident from FIG. 1 the cathode current 13 passes the block 9.This block comprises devices for measuring the cathode current and canbe constituted by a current transformer, a serial resistance over whicha voltage is measured or some other type of device that can indicate thecurrent strength. Alternatively, which can be deduced from that whichhas been written previously, the body/helix current 15 and the collectorcurrent 14 can be measured and thereafter added. The added valuescorrespond to the cathode current 13. If the TWT is provided with morecollectors then the different collector currents have to be added ofcourse.

As the collector current, in travelling wave tubes which work well, isseveral times larger than the body current, it is sufficient in manyapplications to only measure the collector current to determine themodulation of the beam current.

In the cases where the travelling wave tube is used in pulsed operationthe beam current is sampled during the pulse, for instance with a sampleand hold circuit. By comparing the measured/sampled value of the cathodecurrent with a reference value, a difference signal is formed. Thisdifference signal is transferred to the block 11 for control(modulation) of the grid voltage. The beam current will thereby beaffected and, by a suitable choice of "sign" or "phase" of thedifference signal, the modulation of the beam current can becounteracted. The feedback of the cathode current to the grid biasimplies a control loop that strives to bring the modulation of the beamcurrent towards zero.

It is thus possible with the described device to sense the modulation ofthe beam current caused by the mechanical vibrations and, by means ofcontrol of the grid bias of the tube, to reduce this modulation.

As an alternative to measuring the modulation by means of the cathodecurrent, even the high-frequency, amplified signal can be measured. Inthis case as shown in FIG. 2, a part of the signal is coupled, with adirectional coupler or corresponding other device 16, to a quadraturedetector 17 where the high frequency signal is brought back to the baseband. The modulation signal caused by the vibrations can then befiltered out in a band-pass or low-pass filter 18. After comparison 19with a reference value 20, the so-called difference signal betransferred to the block 11 for, as an analogy to what has beendescribed earlier, control of the grid bias. Further it should bementioned that instead of controlling the grid bias it is possible tocontrol a device for amplitude and phase modulation of the input signalto the tube. It should however be mentioned that these embodiments ofthe present invention assume that the travelling wave tube works with asufficiently low input signal so that the tube is not saturated.

The invention is not limited to the described embodiments, but may bevaried within the scope of the appended claims.

What is claimed is:
 1. A method for reducing modulation of a beamcurrent of a travelling wave tube caused by mechanical vibrations,comprising the steps of:measuring the modulation of the beam current;comparing the measured modulation of the beam current with a referencevalue and forming a difference signal based thereon; and adjusting agrid bias of the travelling wave tube based on the difference signalsuch that the measured modulation of the beam current is opposed;wherein the step of measuring the modulation of the beam currentincludes the step of measuring a cathode current of the traveling wavetube.
 2. A method for reducing modulation of a beam current of atravelling wave tube caused by mechanical vibrations, comprising thesteps of:measuring the modulation of the beam current; comparing themeasured modulation of the beam current with a reference value andforming a difference signal based thereon; and adjusting a grid bias ofthe travelling wave tube based on the difference signal such that themeasured modulation of the beam current is opposed; wherein the step ofmeasuring the modulation of the beam current includes the step ofmeasuring a collector current of the traveling wave tube.
 3. A methodfor reducing modulation of a beam current of a travelling wave tubecaused by mechanical vibrations comprising the steps of:measuring themodulation of beam current; comparing the measured modulation of thebeam current with a reference value and forming a difference signalbased thereon; and adjusting a grid bias of the travelling wave tubebased on the difference signal such that the measured modulation of thebeam current is opposed; wherein the step of measuring the modulation ofthe beam current includes the step of measuring a modulation of a highfrequency signal that has been amplified in the travelling wave tube. 4.A device for reducing modulation of a beam current of a travelling wavetube caused by mechanical vibrations, comprising:means for measuring themodulation of the beam current and for forming a signal that representsa difference between the measured modulation and a reference value; andmeans for changing a grid bias of the travelling wave tube based on thesignal, whereby the beam current is affected such that the measuredmodulation is opposed; wherein the means for measuring the modulation ofthe beam current comprises means for measuring a cathode current of thetravelling wave tube.
 5. The device of claim 4, wherein the cathodecurrent is measured by adding a collector current and a body current ofthe travelling wave tube.
 6. The device of claim 4, wherein the meansfor measuring the cathode current includes at least one currenttransformer.
 7. The device of claim 4, wherein the means for measuringthe cathode current includes means for measuring an electrical voltagedrop across a serial resistance.
 8. A device for reducing modulation ofa beam current of a travelling wave tube caused by mechanicalvibrations, comprising:means for measuring the modulation of the beamcurrent and for forming a signal that represents a difference betweenthe measured modulation and a reference value; and means for changing agrid bias of the travelling wave tube based on the signal, whereby thebeam current is affected such that the measured modulation is opposed;wherein the means for measuring the modulation of the beam currentincludes means for measuring a modulation of a high-frequency signalthat has been amplified in the travelling wave tube.
 9. A device forreducing modulation of a beam current of a travelling wave tube causedby mechanical vibrations, comprising:means for measuring the modulationof the beam current and for forming a signal that represents adifference between the measured modulation and a reference value; andmeans for changing a grid bias of the travelling wave tube based on thesignal, whereby the beam current is affected such that the measuredmodulation is opposed; wherein the means for measuring the modulation ofthe beam current comprises means for measuring a collector current ofthe travelling wave tube.
 10. The device of claim 9, wherein the meansfor measuring the collector current includes at least one currenttransformer.
 11. The device of claim 9, wherein the means for measuringthe collector current includes means for measuring an electrical voltagedrop across a serial resistance.